Knock Knock sound left front wheel

Occasionally I get a loud Knock Knock from the left wheel when I accelerate from a stopped position. The sound is loud and rythmic just like someone giving two good knocks to a wooden front door. Any ideas? Occasionally it will happen on a turn.

Sounds like the ball joint. Raise the van with a jack so the front wheel is just an inch or so high. Use a pry bar of some sort and put it under the tire, then lift the bar to pry the wheel inward as you do. The wheel should not yield to this effort and if it does, your ball joint is bad. I've never had anything else make that distinct 'knock knock' sound you describe. Loose/worn suspension points such as the rubber encased bolts on the swing arm might do it, but those are easy to spot visually and most people describe that sound as a rattle noise.